diff --git a/app/build.gradle b/app/build.gradle
index d16c98c..22f89d9 100644
--- a/app/build.gradle
+++ b/app/build.gradle
@@ -81,11 +81,11 @@ android {
         freeCompilerArgs = freeCompilerArgs + "-opt-in=kotlin.RequiresOptIn"
     }
     compileOptions {
-        sourceCompatibility JavaVersion.VERSION_1_8
-        targetCompatibility JavaVersion.VERSION_1_8
+        sourceCompatibility JavaVersion.VERSION_11
+        targetCompatibility JavaVersion.VERSION_11
     }
     kotlinOptions {
-        jvmTarget = "1.8"
+        jvmTarget = JavaVersion.VERSION_11
     }
     buildFeatures {
         compose true
@@ -214,7 +214,11 @@ dependencies {
     implementation "androidx.activity:activity-compose:1.8.2"
     implementation 'androidx.appcompat:appcompat:1.7.0-alpha03'
     testImplementation "junit:junit:4.13.2"
-    androidTestImplementation "androidx.test.ext:junit:1.1.3"
-    androidTestImplementation "androidx.test.espresso:espresso-core:3.4.0"
+    androidTestImplementation "androidx.test.ext:junit:1.1.5"
+    androidTestImplementation "androidx.test.espresso:espresso-core:3.5.1"
 
+    // mockito
+    testImplementation "org.mockito:mockito-core:5.11.0"
+    testImplementation 'org.mockito:mockito-junit-jupiter:5.11.0'
+    testImplementation "org.mockito.kotlin:mockito-kotlin:5.2.1"
 }
diff --git a/app/src/test/java/me/ash/reader/infrastructure/rss/RssHelperTest.kt b/app/src/test/java/me/ash/reader/infrastructure/rss/RssHelperTest.kt
new file mode 100644
index 0000000..ab4bf5b
--- /dev/null
+++ b/app/src/test/java/me/ash/reader/infrastructure/rss/RssHelperTest.kt
@@ -0,0 +1,74 @@
+package me.ash.reader.infrastructure.rss
+
+import android.content.Context
+import kotlinx.coroutines.CoroutineDispatcher
+import okhttp3.OkHttpClient
+import org.junit.Assert
+import org.junit.Before
+import org.junit.Test
+import org.junit.runner.RunWith
+import org.mockito.Mock
+import org.mockito.junit.MockitoJUnitRunner
+import org.mockito.kotlin.mock
+
+const val enclosureUrlString1: String = "https://example.com/enclosure.jpg"
+const val enclosureUrlString2: String = "https://github.blog/wp-content/uploads/2024/03/github_copilot_header.png"
+const val imageUrlString: String = "https://example.com/image.jpg"
+const val enclosureHtmlCase1: String = """
+        <enclosure url="$enclosureUrlString1" type="image/jpeg"/>
+        <img src="$imageUrlString"/>
+    """
+const val enclosureHtmlCase2: String = """
+        <img src="$imageUrlString"/>
+        <enclosure url="$enclosureUrlString1" type="image/jpeg"/>
+        <img src="$imageUrlString"/> 
+    """
+const val enclosureHtmlCase3: String = """
+        <img src="$imageUrlString"/>
+        <enclosure url="$enclosureUrlString2" type="image/png"/>
+        <img src="$imageUrlString"/> 
+    """
+const val imageHtmlCase1: String = """
+        <img src="$enclosureUrlString1"/>
+        <img src="$imageUrlString"/> 
+    """
+const val imageHtmlCase2: String = """
+        <img src="$imageUrlString"/> 
+        <img src="$enclosureUrlString1"/> 
+        <img src="$enclosureUrlString1"/> 
+    """
+
+@RunWith(MockitoJUnitRunner::class)
+class RssHelperTest {
+
+    @Mock
+    private lateinit var mockContext: Context
+
+    @Mock
+    private lateinit var mockIODispatcher: CoroutineDispatcher
+
+    @Mock
+    private lateinit var mockOkHttpClient: OkHttpClient
+
+    private lateinit var rssHelper: RssHelper
+
+    @Before
+    fun setUp() {
+        mockContext = mock<Context> { }
+        mockIODispatcher = mock<CoroutineDispatcher> {}
+        mockOkHttpClient = mock<OkHttpClient> {}
+        rssHelper = RssHelper(mockContext, mockIODispatcher, mockOkHttpClient)
+    }
+
+    @Test
+    fun testFindThumbnail() {
+        Assert.assertNull(rssHelper.findThumbnail(""))
+        Assert.assertNull(rssHelper.findThumbnail(" "))
+        Assert.assertNull(rssHelper.findThumbnail(null))
+        Assert.assertEquals(enclosureUrlString1, rssHelper.findThumbnail(enclosureHtmlCase1))
+        Assert.assertEquals(enclosureUrlString1, rssHelper.findThumbnail(enclosureHtmlCase2))
+        Assert.assertEquals(enclosureUrlString2, rssHelper.findThumbnail(enclosureHtmlCase3))
+        Assert.assertEquals(enclosureUrlString1, rssHelper.findThumbnail(imageHtmlCase1))
+        Assert.assertEquals(imageUrlString, rssHelper.findThumbnail(imageHtmlCase2))
+    }
+}